1. Field of the Invention
The present invention relates to diaphragms for acoustic speakers or transducers, and more specifically, to diaphragms coupled to lightweight supports that impose soft boundary conditions on the diaphragm.
2. Description of the Prior Art
Many lightweight audio sound generators or loudspeakers provide good performance at high frequencies. Low frequency loudspeakers generally are large and heavy, and require high power inputs.
In extremely noisy areas, low frequency sound generation is needed to reduce the overall noise level by application of anti-noise (sound applied 180 degrees out of phase). In space launch vehicles, for example, the preferred method for absorbing the high sound pressure level low frequency noise in the payload fairing area is to include thick aluminum plates in the structure itself. If a lightweight low-frequency sound generator were available for active noise control, these heavy fairings could be replaced by much lighter structures with only enough mass for structural support. Clearly, other vehicles (aircraft, ground vehicles, and ships) and other noisy machinery applications could also benefit from the availability of lightweight sound generators.
The typical low-frequency audio sound generator (i.e. loudspeaker) consists of two key components: an actuator and a diaphragm. In the typical loudspeaker the actuator, which transforms the input electrical energy into displacement and force, is an electromagnetic voice coil. The displacement generated by the actuator is applied to the vibrating diaphragm or cone, which acts as a mechanical lever or piston to increase the volume displacement and hence increase the efficiency of radiation. In order to produce high output at low frequencies, the voice coils must be of relatively high mass. In aerospace applications where weight is a crucial expense, the use of such loudspeakers can become prohibitive. Other sound generators have been devised, but all have serious limitations on their range of applicability. Horn and buzzer type actuators can be designed which are light-weight and capable of low frequency use, however their narrow-band nature and poor controllability limits their use to a narrow range of applications.
One approach to reducing mass in a conventional loudspeaker design has been to use lower-mass actuators than the electromagnetic voice coil. Alternative lower-mass actuators exist, such as piezoelectric monomorphs and bimorphs. These actuators can deliver reasonable displacement, but in previous configurations when coupled to conventional diaphragms in air they have failed to produce the combination of force and displacement needed at low frequencies.
Polymer speakers have been successful in high frequency applications, but have not been capable of delivering the high displacement levels required for low frequency use.
One novel method for producing low frequency acoustic vibrations in air using a polymer acoustic diaphragm is discussed in U.S. patent application Ser. No. 60/208,323, filed on Jun. 1, 2000.
These loudspeakers share similar simple boundary conditions at the edge of the diaphragmxe2x80x94the diaphragm typically is either simply supported (i.e. a drum head) or attempts to approach the free boundary condition (i.e. a piston).
The limitations of current acoustic technology are illustrated by the standard equations are available in textbooks for sound radiation from sources. For a piston source a surface displacement xcex94x will generate a sound pressure given by P=2 xcfx80f Z xcex94x G(S), where P is the sound pressure level, Z is the appropriate acoustic impedance, and G(S) is the function of the separation distance. The acoustic impedance Z can be written as Z=xcfx81aircair, where xcfx81air is the density and Cairis speed of sound in the air surrounding the membrane. The sound pressure level in air can therefore be expressed as:                     SPL        =                  20          ⁢                      xe2x80x83                    ⁢                      Log            ⁡                          (                                                                    2                    ⁢                                          xe2x80x83                                        ⁢                    π                    ⁢                                          xe2x80x83                                        ⁢                                          ρ                      air                                        ⁢                                          c                      air                                        ⁢                    Δ                    ⁢                                          xe2x80x83                                        ⁢                                          xG                      ⁡                                              (                        S                        )                                                                              ⁢                                      xe2x80x83                                                                    20                  ⁢                                      xe2x80x83                                    ⁢                  μ                  ⁢                                      xe2x80x83                                    ⁢                  Pa                                            )                                                          Equation        ⁢                  xe2x80x83                ⁢                  (          1          )                    
where 20 xcexcpa (20xc3x9710xe2x88x926 Pascals) is the standard reference pressure used for air. The G(S) term in Equation (1) is defined as                               G          ⁡                      (            S            )                          =                  2          ⁢                      xe2x80x83                    ⁢          sin          ⁢                      {                                                                                π                    ⁢                                          xe2x80x83                                        ⁢                    S                                    λ                                ⁡                                  [                                                                                    (                                                                                                            (                                                              a                                S                                                            )                                                        2                                                    +                          1                                                )                                                              -                    1                                    ]                                            ,                                                          Equation        ⁢                  xe2x80x83                ⁢                  (          2          )                    
where xcex is the wavelength, a is the radius of the piston source or membrane, and the separation distance S is the axial distance from the membrane to the point at which the sound pressure level is calculated or measured.
Equation (2) is applicable only to the pressure on the axis of a disk array, and contains nulls and reinforcements not experienced at off-axis locations. An alternative expression which ignores the local nulls particular to the on-axis response can be obtained by using the simple farfield distance dependence:                               G          ⁡                      (            S            )                          =                  A                      λ            ⁢                          xe2x80x83                        ⁢            S                                              (        3        )            
where A is the array area (cross sectional area of the piston).
The acoustic impedance Z of Equation (1) is typically the real part of the radiation impedance, which results in the net radiated energy. Alternatively, if the result desired is an envelope of the axial response, then the value used for Z in Equation (1) is twice the magnitude of the total radiation impedance. One expression for the radiation impedance is:                               Z          ⁡                      (            X            )                          =                              (                          1              -                                                2                  ⁢                                      xe2x80x83                                    ⁢                                                            J                      1                                        ⁡                                          (                      X                      )                                                                      X                                      )                    +                      i            ⁡                          (                                                2                  ⁢                                      xe2x80x83                                    ⁢                                                            H                      1                                        ⁡                                          (                      X                      )                                                                      X                            )                                                          Equation        ⁢                  xe2x80x83                ⁢                  (          4          )                    
were J1 is the first order Bessel function, H1 is the first order Struve function (a well known mathematical function in acoustics), and X=2xcfx80a/xcex.
To illustrate the characteristics of currently available loudspeakers, FIG. 1 compares the calculated values for the generated sound pressure level along the axis of a 28 cm diameter circular membrane clamped at its peripheral edge (resembling the behavior of a piston type source). The conditions used are for nominally 1 micron displacement, with an axial measurement of sound pressure level at a separation distance of 25 cm from the face. At frequencies below 1.5 kHz, the distance of 25 cm is in the acoustic farfield of the radiator.
FIG. 1 illustrates that while the source level (sound pressure level) is very large at medium to high frequencies, it falls off rapidly at low frequencies. To achieve high sound pressure levels at frequencies below 200 Hz for pistons or conventionally clamped diaphragms, one must use much larger displacements than the one micron value used above. In order to produce the required greater displacements, stiffer and more massive diaphragms, higher input energies and higher actuator force levels are necessary.
Conventional loudspeakers rely on the stiff frame (to which the diaphragm is attached) to ensure that the dynamic force opposing the axial displacement of the diaphragm is contributed primarily by the membrane, rather than by the frame. The tension, edge compliance, and material properties of the membrane are critical to good performance. The tension must generally be even all around to produce a reasonably uniform sound output at low frequencies. Additionally, some conventional membrane-based acoustic projectors must be adjusted at frequent intervals to ensure that the tension does not drop too low.
In order to illustrate the advantages of the present invention compared to conventional loudspeakers, a loudspeaker was fabricated using a polymer acoustic membrane as a diaphragm, and the peripheral edge was clamped to a stiff circular frame, (28 cm in diameter). A xe2x80x9cThunderxe2x80x9d type piezoelectric monomorph bender actuator was attached to the diaphragm-frame assembly, and located so a free end of the bender actuator was in contact with a face of the diaphragm. As a voltage was applied to the actuator, the actuator bent proportionately to the applied voltage, moving the free end of the piezoelectric actuator axially, and deflected the diaphragm surface in an axial direction.
FIG. 2 illustrates the sound pressure levels measured using a calibrated microphone at a point 25 centimeters above the center of the diaphragm. In this test, the applied voltage was 200 volts. FIG. 4 also plots the predicted sound pressure level for an ideal piston source based on a one micron piston displacement and 200 volts peak.
Projectors with acoustic membranes having clamped-edge boundary conditions of similar size for various materials in various frames will exhibit behavior similar to that shown in FIG. 2. Below 150 Hz the performance typically decreases with a slope of 40 to 60 dB per decade. Performance above 150 Hz typically approaches a relatively constant SPL value (85 to 95 dB for the example shown in FIG. 2 at 200 V drive and 25 cm distance), presumably due to either the system reaching a dense wavenumber distribution region and/or a force-limited condition. The mounting arrangement and diaphragm tension may be adjusted to introduce some resonant behavior in the low frequency region, however these contributions are typically small and often degrade performance in nearby frequency bands, reducing broadband performance.
It is apparent that none of the current loudspeakers meet the need for lightweight acoustic projectors with good low frequency performance in air. These are particularly needed for active noise control systems.
An object of the invention is to provide a lightweight loudspeaker with good low frequency sound pressure levels.
An object of the invention is to provide a lightweight loudspeaker which produces broadband performance over mid and low frequencies.
Another object of the invention is provide an acoustic projector for use in active control systems.
The invention described herein is a low-mass, light-weight sound generator with particularly good performance at mid and low audio frequencies. It is expected to find principal use in applications where mass is of crucial importance. It is capable of delivering the high acoustic levels, as required for sound generation or active sound control. The present invention uses a generally planar acoustic membrane as a diaphragm, the diaphragm being in tension and attached at its outer edge to a support frame which provides a soft boundary condition to the acoustic diaphragm. Preferably, an actuator transmits axial displacement to the acoustic diaphragm in response to an applied voltage, thereby exciting resonances in the diaphragm and generating high sound pressure levels in the air in front of the diaphragm.